During preseeded install, apt-cacher is not yet available.

roles/educontainer/tasks/setup.yml

@@ -16,7 +16,7 @@
 - name: debootstrap base system
   command:
     cmd: >
-      debootstrap 
+      debootstrap
       --include={{ cont_packages | union(cont_packages_extra) | join(',') }}
       --components=main,contrib,non-free stable
       {{ contname }}00 http://deb.debian.org/debian
@@ -24,7 +24,7 @@
     chdir: /var/lib/machines/
     creates: /var/lib/machines/{{ contname }}00
   environment: 
-    http_proxy: http://localhost:3142
+    http_proxy: "{{ '' if run_in_installer|default(false) else 'http://localhost:3142' }}"
   notify: enable and restart containers
 
 - name: provide complete apt sources